The present invention relates to an electrode for living body which is mounted on the skin of a patient to detect the electric signals generated by the physiological electric phenomena taking place in a living body such as electrocardiographic signals, electromyographic signals electroencephalographic signals and the like. In particular, the invention relates to the material of such electrodes for living body capable of very reliable performance.
At the present time, there is a large demand for the measurement and recording of the physiological electric phenomena such as electrocardiograms and electroencephalograms not only in the medical science but also in the fields of biophysics and bioengineering. In these measurements, the role played by the electrodes is very important in order to detect the weak electric signals generated in the living body and to serve as a medium to transmit the signals to the measuring instruments.
Various types of electrodes are used in recent times in order to obtain reliable performance which depends largely upon the construction and material of the electrode. It is the basic requirement that the fluctuation of the electrode potential caused by some phenomena other than the objective electric signals from the living body and the generation of noise can be minimized in order to detect the electric signals accurately and transmit the signals to the measuring instrument when the electrode is mounted on the skin of the living body because the electric signals generated in a living body cannot be so high as exceeding 1 mV or so at highest.
Living bodies are filled with a tissue liquid which can be simulated with the physiological sodium chloride solution, i.e. aqueous solution of sodium chloride in a concentration of 0.9%. Therefore an electrode mounted on a living body to detect the electric signals from the living body and to transit the signals can be simulated approximately by the electrode immersed in an aqueous electrolyte solution which is a 0.9% sodium chloride solution to detect the electric signals from a living body passing through the electrolyte solution and transmit them.
In this case, accurate detection and transmission of the electric signals from a living body require electrode characteristics of the stability in the electrode potential, small impedance of the electrode and the absence of the noise voltages. The fundamental condition to satisfy these requirements is the electrode reaction with good reversibility taking place on the electrode which behaves as a non-polarizable electrode.
Notwithstanding the above requirements, conventional electrodes for living body made of metals such as gold, platinum, silver, tungsten, molybdenum, copper, stainless steel and the like suffer from very large fluctuation in the electrode potential amounting to several tens or hundreds of millivolts in most cases. Thus metal electrodes for living body are never satisfactory for practical use in accurately detecting and transmitting the electric signals from a living body.
In a metal electrode in contact with the liquid in living tissue, i.e. an electrolyte solution, a stable and reversible electrode reaction can hardly be expected to establish the potential of the metal electrode against the solution due to the lack of the free exchange of ions and electrons between the phases of the metal electrode and the electrolyte solution. This situation leads to the unstable potential of the electrode and, particularly immediately after mounting of the electrode to the skin of a living body, to a very remarkable fluctuation in the potential to be stabilized at a relatively constant level only after a lapse of considerable length of time to a great drawback in practical use of the electrode.
As a remedy for the above described defects in metal electrodes for living body, an improved electrode material of silver chloride/silver is known. The performance of this type of the electrode is based on the mechanism that the electrode reaction to establish the electrode potential expressed by the equation AgCl+e.sup.- .revreaction.Ag+Cl.sup.- takes place reversibly and with stability so that the exchange of electrons and ions proceeds freely at the interface between the electrode and the electrolyte solution by this reaction giving rise to a relatively stable characteristics of the electrode potential without remarkable polarization.
The silver chloride/silver electrodes above mentioned are prepared typically either by the method of electrolysis conducted in a sodium chloride solution with a silver plate as the anode to deposit a film of silver chloride on the surface of the silver plate or by the method of press molding of a mixture of metallic silver powder and silver chloride powder into a shape of an electrode plate.
The former method, by which most of the commercially available silver chloride/silver electrodes are prepared, has a great difficulty in obtaining uniform film of silver chloride on the surface of the silver plate so as that a stable characteristics of the electrode potential can hardly be expected.
On the other hand, the latter method for the preparation of silver chloride/silver electrodes is disadvantageous from the practical standpoint because of the handling of chemically unstable silver chloride liable to decomposition or denaturation, especially, under the influence of light necessitating special care in the storage and handling of the electrodes.
In addition, the silver chloride/silver electrodes are economically disadvantageous in any way because of the expensiveness of silver and silver chloride as the materials and not suitable as disposable electrodes of wide prevalence with rapidly growing demand in recent years. Thus the electrodes of this type are also excluded from practical use with economy.